A dormer is a roofed structure, often containing a window, that projects vertically beyond the plane of a pitched roof. A dormer window (also called dormer) is a form of roof window. Dormers are commonly used to increase the usable space in a loft and to create window openings in a roof plane. A dormer is often one of the primary elements of a loft conversion. As a prominent element of many buildings, different types of dormer have evolved to complement different styles of architecture. When the structure appears on the spires of churches and cathedrals, it is usually referred to as a lucarne.